1. Field of the Invention
This invention relates to amplifiers and more specifically to an amplifier input stage.
2. Description of Related Art
The amplifier shown in FIG. 1a operates as illustrated in FIG. 1b, showing the input buffer portion of FIG. 1a with the associated current flows. At a balanced condition, i.e. no input signal, ideally there is no current through resistor R.sub.E. If a signal amplitude at the +IN terminal increases (rises) by a value .DELTA.V.sub.in the current through transistor Q5 decreases by .DELTA.i, as does the current through transistor Q2. At the same time, the current through transistors Q4 and Q6 increases by .DELTA.i. Thus the current through resistor R.sub.E is equal to 2.DELTA.i=.DELTA.V.sub.in /R.sub.E. Note that the gain of the input buffer shown in FIG. 1b is g.sub.m =1/R.sub.E. Current .DELTA.i also appears through resistors R1 and R2 in FIG. 1a.
Thus a net current 2.DELTA.i (see FIG. 1a) charges the capacitances C1 and C2 at high impedance gain node G. Under large signal conditions the change in voltage at the high impedance gain node G (which is also the input terminal of the amplifier output stage here designated OP) is dependent on how fast the voltage changes at the emitters of transistors Q9 and Q12.
The current provided to the emitters of respectively transistors Q9 and Q12 is relevant to charging and discharging capacitances C1 and C2 (which include both the compensation capacitors and the parasitic capacitances at point G), because the maximum sourcing current at the collector of transistor Q10 is equal to the .beta. of transistor Q10 times current I.sub.O from current source I5. Similarly, the maximum sinking current at the collector of transistor Q11 is equal to the .beta. of transistor Q11 times the current I.sub.O from current source I6.
Therefore in order to achieve a higher slewing rate, i.e. a fast response of the output signal to a fast changing positive going or negative going input signal, one must provide more current. This requires increasing the current sourcing/sinking capabilities of current sources I5 and I6, which disadvantageously requires a larger device area and also involves consuming higher idling power, due to the higher quiescent current.
Thus the technical problem is that it is difficult to respond quickly to such a high slew rate signal in an amplifier of the type of FIG. 1a without undesirably providing a high quiescent current to the base terminals of the transistors Q10 and Q11. Such high quiescent current is undesirable both because of the need to provide large size devices to drive such a current and because a high current means high power dissipation.